AppChooser is not modal anymore

Remove modality flag so that AppChooser won't block input on other
windows making interacting with the launcher possible, in order to
launch applications or click the button to hide AppChooser.
Calculate available geometry

Evey time the screen geometry changes we recalculate the available
geometry which will be used by shell components such as AppChooser.
Rename desktop shell protocol file

Rename to a more appropriate name.
Request to change surface position

Some surfaces such as indicators' menus and AppChooser need to
force their position. This is not supported by Wayland, clients
create surfaces and only specify the size, hence we need a new
request to force the position.